This invention relates to an improved process for making an apertured nonwoven fabric from a fibrous web.
U.S. Pat. No. 3,485,706 discloses the basic process of impinging fine columnar streams onto a fibrous web supported on an apertured support to convert the web by fiber entanglement into an apertured nonwoven fabric. The apertures in the fabric correspond to solid portions of the apertured support on which the fibrous web is positioned during the impingement process. When the apertured support is a woven wire screen, the apertures in the fabric correspond to knuckles in the screen. Among the possibilities for operation disclosed in this patent is the posibility of using a woven-wire screen such as shown in FIG. 14 of the patent wherein the screen is asymmetrical in the sense that the wires running in one direction have a greater crimp and thereby form a higher knuckle than the wires running in the transverse direction. Other types of screens are shown in FIGS. 20-23 of the patent.
Another possibility for operation is to carry out the impingement step using equipment such as shown in FIG. 1 of the patent first in one direction on one face of the web and then in the transverse direction on the same face of the web to form the fabric.
Another possibility as shown in FIG. 2 of the patent is to carry out the impingement step using a series of banks of fine columnar streams exerting increasing impact force on only a single face of the web to form the fabric. In this single stage treatment it was found that using an asymmetrical woven-wire screen with the wires forming the higher knuckles running in the cross (transverse) direction of passage of the web beneath the streams of liquid increased the tensile strength of the resultant fabric.
Still another possibility as shown in FIG. 40 of the patent is to carry out the impingement step in two stages, first on one face of the fibrous web and then on the opposite face of the fibrous web.
The two-stage impingement process has been operated using as the apertured support for the web in the second stage an asymmetrical woven-wire screen wherein the wires forming the higher knuckle run in the cross direction relative to the direction of passage of the fibrous web beneath the fine columnar streams of liquid. In this process, the apertures in the fabric correspond at least to the higher knuckles in the screen. For very low area weight webs such as 0.8 oz/yd.sup.2 (27.1 g/m.sup.2), there may be additional apertures in the resultant fabric corresponding to the lower knuckles formed by the transverse wires of the screen. In this specific process, the desire arose to increase efficiency of operation, i.e., to increase the strength of the fabric without using more liquid; or to get the same strength in the fabric by using less liquid; or to get equivalent strength using a lower area weight web, which would lead to an increase in the rate of production of the web.